<html> <head> <meta http-equiv="content-type" content="text/html; charset=UTF-8"> </head> <body bgcolor="#FFFFFF" text="#000000"> I have been attempting to get passed through sound cards working and have been encountering issues. Apart from the extra complication of windows, I've been testing with a real-time audio app like yoshimi, and I get audio lag/glitches. I've been tracking down the causes of these glitches and have found several things that contribute. So far, on two setups I have, I've found: <ul> <li>>1ms long SMIs on a GA-990FX motherboard, caused exclusively by parallel disk writes (I am running a 5-disk array on it) </li> <li>The linux radeon driver (on the host) occasionally locking up a core for 10-35ms at a time (I think this has been fixed since the 4.19 series) </li> <li>When console messages get sent to a serial port, it locks up a core for the duration of the message. (This is a known problem that is still being worked on)</li> <li>qemu's emulated (uhci) USB seems to hold up a vCPU for a few ms somewhere in the guest's interrupt handler</li> </ul> I've been able to track these down with some combination of - the SMI (hwlat) detector in ftrace - the irqsoff tracer in ftrace - cyclictest - guesswork I have found workarounds for the above, however I am still having trouble with one box - I get glitches every minute or two. My investigation is so far stuck at hits from the irqsoff tracer that contain lines like this: <tt>alsa-sin-31611 1d... 0us#: _raw_spin_lock_irq</tt><tt> </tt><tt>alsa-sin-31611 1d... 4448us : snd_pcm_group_unlock_irq</tt><tt> </tt> So, questions: - when I get output like the above from an irqsoff trace, how do I probe further? - are there any other tools that could help with debugging these sorts of problems? - I understand that for IRQs to get routed properly from host to guest, the device needs to have a per-device (per-function?) IRQ status bit that can be used to aid in routing. How do I confirm that the device in question has this bit? - how do I check that IRQs are being routed to the guest optimally? What do I watch out for? How do I check if it's using irqfd? - does MSI really help? Will it save any more more than, say, 100us in IRQ response latency? (I am able to swap in a sound card that works; I have a cheap USB dongle that hasn't given me any problems yet. I still want to see what options I have though.) </body> </html>